Optical Dispersion, Band-Gap Engineering and Structural Disorder in Er₂O₃/Er₂O₃ Nanoparticle-Doped Zinc Borotellurite Glasses for Photonic Applications.

Zinc borotellurite glasses with nominal composition {[TeO₂]₀.₆₅ (B₂O₃)₀.₃₅]₀.₆₅ (ZnO)₀.₃₅}₁₋ₓ (Er₂O₃ / Er₂O₃ NPs)ₓ (x = 0.005–0.030) were synthesised by the melt-quenching technique and systematically investigated to compare the effects of conventional Er₂O₃ and Er₂O₃ nanoparticles on optical dispersion, band-gap characteristics and structural disorder. X-ray diffraction confirmed the amorphous character of both glass series, while transmission electron microscopy and energy-dispersive X-ray spectroscopy revealed well-dispersed Er₂O₃ nanoparticles with sizes of approximately 10–20 nm and confirmed the presence of the constituent elements. Fourier-transform infrared spectra indicated concentration-dependent modifications of the TeO₄/TeO₃ and BO₃/BO₄ structural units, consistent with changes in the local bonding environment. The density increased with Er₂O₃ incorporation, accompanied by a reduction in molar volume, with the nanoparticle-doped glasses exhibiting higher densities than their conventional counterparts. The optical absorption spectra showed the typical Er 3+ 4f-4f transitions, such as the 4I 15/2 → 4I 13/2 transition around 1.53μm. The refractive index increased with dopant concentration, and the glasses doped with nanoparticles showed consistently higher values. Tauc analysis showed a gradual reduction of the indirect optical band gap from 2.954 to 2.568 eV for the conventional Er 2 O 3 series and from 3.061 to 2.390 eV for the nanoparticle series, with an increase in Urbach energy from 0.315 to 0.675 eV and 0.340 to 0.800 eV, respectively. The simultaneous band-gap narrowing and Urbach tail broadening suggest dopant-induced modifications in the local bonding environment and increased structural/electronic disorder, with effects being more pronounced for nanoparticle incorporation. Overall, Er₂O₃ nanoparticle doping provides an effective route for tailoring refractive-index dispersion and the optical absorption edge of zinc borotellurite glasses, highlighting their potential for near-infrared photonic and fibre-optic applications.